Furnace for neutral heating of metal with neutral preheat



July 22, 1958 F. A. RuscANo '2,844,365

mames FOR NEU TRAL HEATING OF METAL WITH NEUTRAL PREHEAT 5 Sheets-Sheet2 Filed April 9. 1954 INVENTOR. F.A.RUSCIANO BY W.

A TO RNEY United States Patent O 2,s44,36s u FURNACE FOR NEUTRAL HEATNGOF METAL WITH NEUTRAL PREHEAT Frank A. Rusciano, New York, N. Y.,assignor to Metallu'gical Processes Co., Newark, N. J., a corporauon ofNew Jersey Application April 9, 1954, Serial No. 422,013

2 Claims. (Cl. 266--5) This invention relates to a method for producingrapid and efficient heating of metals to an elevated temperature undernon-scaling conditions and to a furnace structure by which such methodmay be practiced.

In a copending application of F. A. Rusciano, Serial No. 422,074, filedApril 9, 1954, and entitled Neutral Heating With controlled Preheat, acontinuous furnace is disclosed in which the initial heating, up to arange of temperatures between about 1000 F. and l500 F., is conductedrapidly in a direct combustion atmosphere having high heating Capacityand a composition which, at higher temperatures, or for prolongedheating periods within the stated range, would be highly scaling tometal, the heating up to the final temperature, which may be as high as2500 F., being conducted in an atmosphere which is non-oxidizing to themetal or even deoxidizing, if desired, to the slight oxidatiou which mayoccur in the initial heating. In the method of the above-mentionedapplication the heating atmosphere employed in the initial heating isproduced by the combustion, in a portion of the heating chamber, of amixture of air and fuel in such proportion as to efiect substantiallycomplete combustion of the fuel, and the non-scaling or deoxidizingatmosphere employed in the final heating is' produced by reaction, inanother portion of the heating chamber, of an air-fuel mixture having alarge excess, of the order of 100%, in the amount of fuel required forcomplete combustion. The reaction products produced ,in this latterportion of the furnace heating chamber contain a large amount ofpotential heat which is made available by secondary combustion of thisrich atmosphere gas externally of the work heating chamber but in heattransfer relation thereto. Specifically this secondary burning iscarried out in a supplemental heating chamber disposed above the mainheating chamber and separated therefrorn by a relatively thin arch ofgood heat conductive material.

The present invention relates to the same general process of heating themetal to a predetermined initial ternperature by the heat of reactionproduced by the complete combustion of an' air-fuel mixture produceddirectly in the initial heating chamber, and to its final temperature ina non-scaling atmosphere produced in the final heating chamber, butdiffers from the process of the aforesaid application in that the metalis protected against oxidation during this initial heating bysegregating the so-called oxidizing atmosphere by a layer of a neutralatmosphere derived from the atmosphere in which the final heating iscarried out. In the instant inventicn the secondary combustion chamberof the aforesaid application, for utilizing the potential heat containedin the non scaling atmosphere gases, has been eliminated and these gasesare employed, by the addition of supplemental air, to produce the heatrequired in the initial heat chamber. This greatly increases theOperating efficiency of the furnace and reduces the structural costthereof, these being among the objects of the invention.

&844565 Ptente'd July 22, 1958 The production of the non-scalingatmosphere ern-` ployed in the final heat zone for protectng the metalwhen it is at elevated temperatures is fully described in a copendingapplication of F. A. Rusciano and H. J. Ness, Serial No. 347,7l6, filedApril 9, 1953,` and entitled Method and Apparatus for ProducngControlled Furnace Atmospheres. Briey, this method comprises adjustingthe air-fuel ratio supplied to the burners so that the reactionproducts, when the reactions are carried to completion, will have CO /COand H O/H ratios of such value that the sum thereof is substantiallyequal to one. The reactions resulting from such a mixture release only asmall percentage of the available B. t. u. of the fuel, of the order of25%, and therefore provide a low heating rate and a restricted uppertemperature limit. These inherent limitations have been overcome in the.processes of each of the previously mentioned applications by furthercombustion of the atmosphere gases out of contact with the work, as forinstance, in radiant tubes passing through the heating chamber or in acombustion chamber partitioned from the heating chamber and in heattransfer relation thereto. These expedients have been commerciallysuccessful but they involve structural disadvantages, particularly inlarge continuous furnaces.

In accordance with the present invention the indirect heating of themetal by a secondary combustion chamber or radiant tubes is dispensedwith and the gases which form the rich non-scaling atmosphere areemployed to directly heat the netal by the secondary combustion thereofin a part of the furnace chamber which, for convenience, may be termedthe initial heat zone. This secondary combustion is carried tocompleton, that is, sutcient air is provided to completely Convert theCO and Hg content of the gases to CO and H O. Such an atmosphere incontact with the work at elevated temperatures is highly scaling andtherefore, in order to prevent such scaling of the work, the combustionis so controlled that the products thereof do not come into contact withthe work while it is above the scaling temperature, that is, above about1000 F. to l F. Above this lower temperature limit and up to surfacetemperatures of 1800 F. to 2000 F. the metal is protected by a layer orblanket of neutral gas, freely fiowing from the hottest to the coolestend of the work. This layer constitutes the combustible fuel for theinitial heat chamber and is gradually consumed as it flows through thechamber by supplying a layer of air thereover in such manner as toeffect burning at the upper surface of the fuel gas layer and out ofcontact with the work, the protective efiect of the free flowing gaslayer being retained over all parts of the work materially above thelower limit of the scaling temperature range. The Work is thus subjectedto the high heat of this complete combustion while being protectedagainst the scaling effect thereof. Above about 2000 F. the work ispassed into the final heat zone where the heating continues up to finaltemperature, which may be from 2300 F. to 2500 F. in the rich neutralgas atmosphere created therein. In the production of this latteratmosphere supplemental heat, over that produced by combustion of therich fuel-air mixture, must be supplied since the flame temperature of asuch a mixture is below the desired metal heating temperature. Thissupplemental heat is provided by preheating the air employed in themixture to a high temperature, preferably around 1000 F. This increasesthe ame temperature of the rich mixture up to approximately that of acompletely combusted atmosphere, that is, up to 2800 F. to 3000 F., andthereby imparts to the final heat zone a heating rate commensurable tothatob tainable with a completely combusted atmosphcre.

The rich neutral atmosphere of the final heat zone is caused to passinto the initial heat zone to form the free flowing blanket therein byregulated draft conditions in the initial heat zone vents, the locationof these vents being such as to draw the neutral gas through the path oftravel of the work and hence to envelop the work With a relatively thicklayer of these neutral gases.

The invention will be more fully understood by reference to theaccompanying drawing in which:

Fig. 1 is a central vertical longitudinal section of a furnace embodyingthe features of the invention;

Figs. 2 and 3 are Vertical transverse sections of the furnace taken onthe lines 2-2 and 3 3, respectively, of Fig. 1;

Fig. 4 is a curve showing the relative scaling condition prevailing inthe initial heat zone in comparison with that normally inherent in acompletely combusted atmosphere; and

Fig. 5 is a schematic View showing the door opening and associatedcontrol mechanism of the furnace.

Reference will'first be made to Figs. 1 to 3 in which a continuousfurnace of the gravity or roll-down type is shown. The furnace iscomposed of refractory brickwork and includes a base 10, forming aninclined floor for the furnace, and the two opposite side walls 11 and12. An arched roof 13 extends over the discharge end section of thefurnace, and with an end wall 14 and the base and side walls 11 and 12,defines the final or high work ternperature heating chamber 15. A secondarched roof 16 extends over the charging end section of the furnace andwith an end wall 17 and the floor and side walls forms the initialheating chamber 18. The end wall 17 is provided with a work chargingslot 19 and the opposite end wall 14 has a similar work discharge slot21, the work W, shown as round bar stock, being conveyed from thecharging slot to the discharge end of the furnace by gravity, rollingupon a series of spaced rails 22, set into the floor of the furnace andcomposed of a material, such as silicon carbide, which is capable ofwithstanding both the heat and the load. Water cooled conduits may alsobe employed for this purpose. It is to be understood, however, that theinvention is susceptible to use in furnaces of other types employing anyconvenient work conveying means.

The bars W come to rest at the discharge end of the furnace against ashouldered portion 23 of the rails 22 and are raised over this shoulderfor discharge through the slot 21 by pneumatically operated levers 24.The bars W may be conveniently spaced from contact with each other byloose rings, not shown, which are passed over the ends of alternate barsas they are loaded into the furnace.

' These spacers` prevent the hot bars from tending to adhere to eachother and thus facilitates their rolling along the sloping rails 22following removal of a bar from the discharge end. Better circulation ofthe hot furnace gases about the bars W, and hence a higher heating rate,also results from this spacing.

The charging and discharging slots 19 and 21 are each provided with adoor 26 and 27, respectively, adapted to be opened and closed bypneumatic cylinders, as will appear in connection with a description ofFig. 5. Interlocks, later to be described, are provided, whereby neitherdoor can be opened until the other door is closed so as to prevent astraight draft through the furnace chamber which might disrupt theprotecting gas blanket formed in the initial heat chamber 18, ashereinafter described.

The high work temperature heat chamber is provided with a series ofburners 28 extending through the opposite side walls 11 and 12. Theseburners are supplied with a very rich mixture of fuel and air,proportioned so as to provide a non-scaling atmosphere in chamber 15.The mixture, as previously indicated, is one which on complete reactionwill produce an atmosphere in which the sum of the CO CO and H O/Hratios is approxirnately one. The actual CO /CO and H O/H ratios willvary with temperature, the respective values at 2000 F. being about .32and .68; at 2150 F. about 0.28 and 0.72; at 2300 F. about 0.26 and 0.74;and at 2500 F. about 0.22 and 0.78. The proper air-fuel mixture forobtaining these ratios is largely independent of the operatingtemperature and controlled primarily by the ratio of molecular carbon tohydrogen in the fuel, Thus a fuel having a C/H molecular ratio of 0.75,such as propane (CaHg) will be restricted to approximately 54% of theoxygen required for complete combustion in order to achieve the above CO/CO and H O/H ratios. With higher C/H ratios, larger proportions of airmay be employed. Substantially all industrial furnace fuels will have aC/H ratio between 0.5 and 1.5 and in practice will produce the desiredCO CO and H O/H ratios with a deficiency of air from about 45% to 50%,that is, with a fuel excess of from to of that required for completecombustion. Curves showing these relationships are disclosed in theaforesaid application of Rusciano et al.

The burners 28, as shown in cross-section in Fig. 2, each 'comprises aburner block 29 of high heat resisting refractory, preferably siliconcarbide. The block is elongated to provide a long tunnel, sufcient topermit the air-fuel reactions to be substantially completed in theburner block. It is restricted slightly at its outer end 31 in order toincrease the pressure within the block and to discharge the reactionproducts at a higher velocity, and is directed upwardly against thearched roof 13 so that the reaction products projected thereby willscrub vigorously against this refractory surface and thus ensure thatthese reactions will be carried to the ntmost completon before theproducts thereof come into contact with the work. This manner of firngalso causes the arch surface to become highly heated and thus aneffective source of radiant heat for the work.

Highly heated air at suitable pressure is supplied to each of the burnerblocks through the vertical passageways 32 which communicate. with anair Conduit 33, one of which extends along each side of the furnacethroughout the length of the final heat chamber 15. Gas or othersuitable fuel, under a predetermined pressure, is fed to the burner by anozzle 34, the relative quantity of air and gas being proportioned bysuitable valves, to produce the desired non-scaling atmosphere inchamber 15.

The amount of air supplied to the burners 28, as stated, is only about50% of that required for complete conibustion of the fuel, or otherwisestated, the nozzles 34 are supplied with substantially a^1GO% excess offuel, and the heat developed from this incomplete combustion is onlyabout 25% of the total available B. t. u. content of the fuel employed.In order to utilize this remaining heat and to protect the work in theinitial heat zone, the atmosphere from chamber 15 is caused to ow freely,from this chamber into chamber 18 through the opening 35 under theeifect of the draft in a set of flues, such as 36, disposed in spacedrelation across the end wall 17, by which the entire furnace gases areexhausted. It will be noted that the forward end of the roof arch 16 isinclined downwardly at 37 towards the charging slot 19 to form arestricted passageway 38 for the gases leaving chamber 18. Thus thedraft effect of flues 36 serves to draw the neutral gas from chamber 15in a straight path from opening 35 to passageway 38, this also being thepath of travel of the work W. Further directional effect is given tothis gas by the continuation of the top, bottom and side walls ofchamber in this same path of travel whereby there is no change ofdirection of these gases in passing into chamber 18 and hence a minimumof agitation. These gases, in passing over the work adjacent the outletof chamber 15, Will be 'cooled to a temperature lower than thatprevailing in the upper part of chamber 18, as will more fully appear,and thus eliminate thernal circulation of these gases upwardly towardsthe roof of chamber 18, thereby retaining this free flowing blanket ofgas adjacent to the floor of the furnace. It should be noted that theVolume of gas delivered by chamber 15 is slightly greater than the totalVolume of air to be added to chamber 18 to burn it to completion,approximately 50% ofthe total air required for complete combustion beingsupplied through burners 28 and the remaining 50% through a series ofslots 39 disposed in spaced relation along the side walls of chamber 18adjacent the arch 16. Only a portion of this air will be entered throughthe slots at the left end of 'chamber 18 and hence the neutral gas, asit enters chamber 18, will have a thickness above the floor of more thanhalf of the depth of the chamber and will thus form a relatively thickblanket over the work at this point.

The air for slots 39is provided by conduits 41 which terminate in'enlarged recesses 42 below the level of the slots 39, impinging upon arefractory brick 43 set in the .furnace Wall. Thisconstructioninterrupts the directional effect of the conduits 41 andthis, together with the large port areas of the entrance slots 39,causes the air to flow slowly into the chamber 18 adjacent the arch roof16. This air being at a lower temperature than the burning gasesimmediately therebelow drifts downwardly into contact with the rich gasfiowing along the base of the chamber, combustion occurring at the areaof contact of the mass of gas and air with the production of an intenseheat -at the upper surface of the protective blanket. As the freefiowing gas mass moves toward the outlet vents 36, it is graduallyconsumed by means of this stratified burning with some intermixing withthe more `completely combusted gas so produced. The rate of dissipationand dilution of the protective blanket may be controlled by the spacingof the air inlet slots 39 and the Volume of air supplied respectivelythereby, and this blanket should not be completely dissipated until itapproaches the loading end of the furnace at a point where the work isbelow its active scaling temperature. The general contour of theprotective blanket is indicated by the dotted line 44.

The work parts W entered through -charging slot 19 may be substantiallyat room temperature and as they pass through chamber 18 they will behe-ated progressively up to the desired top initial temperature which,as stated, may be about 2000 F. The lower limit of the active scalingrange may safely be considered to be about 105G F. If it is assumed thatthe bars reach this lower temperature at point A in their travel, thenthe distribution of the -air entered by slots 39 may be adjusted so thatthe protective blanket is completely dissipated at about this point.Points B to E indicate the position in the travel of the Work when itwill have attained temperatures of about 1400 F., 1700" F., l850 F., and2000 F respectively. At point E where the gases enter chamber 18 theywill be completely neutra to the work and will completely envelope it sothat no scaling can result from the combustion occurring -at theboundary of the gas and air nasses This condition will continue as thefree flowing neutral gas stream moves forward until a point is reached,which, by Way of example, may be in the neighborhood of point C,representing a work temperature of about 1700 F., where inter'minglingof the neutral and oxidizing gases may start to occur in contact withthe work. From this point forward the ratio of neutral to oxidizng gasin contact with the work will gradually increase. However, at point Cthe dilution of the gas blanket will be too slight to produce scale, andas' the dilution gradually increases towards the charging end 6 of thefurna-ce, the work due to its gradually lowered temperature continuestoresist oxidation.

The relative scaling rate occurring in chamber 18 as compared to thatincident to a completely combusted atmosphere will best be understood byreference to the curves of Fig. 4. In this figure curve F indicates in ageneral way the scaling rate of a completely combusted atmosphere over arange of temperatures between 1000 F. and 2000 F. The spacing of thetemperature@ abscissa is in relation to the normal rate of surfaceheating to be expected at each temperature and the area disposed aboveand to the left of curve F is a neasure of the total scale produced inheating up to any given temperature. Curve G represents the generalscaling rate of a combusted mixture containing a 50% eXcess of fuel andthe vertical ordinate or Y axis H represents the zero scaling rate of aneutral combusted mixture containing approximately a excess of fuel.This latter atmosphere is the one employed in chamber 15 and as theprotective 'blanket in chamber 18. If it be assumed that air additionsto chamber 18 are so regulated that above l700 F. there will be nodilution of the protective atmosphere, at 14GO F. a dilutioncorresponding to a 50% excess of fuel, and at 1050 F. a completecombustion of the neutral atmosphere, the oxidation rate in chamber 18will generally follow curve G up to 1400 F. and decrease to zero atabout 1700 F., as indicated by Curve I, the total oxidation obtainablebeing represented by the shaded area 45 to the left of Curve I. Whilethis area indicates that complete absence of oxidation is not obtained,it should be pointed out that the amount of oxidation representedthereby is a relatively small percentage of that produced by a normalcompletely combusted atmosphere in heating up to 2000 F. It appears onlyas a mild discoloration too slight to be properly Classified as scale,and s readily reduced in the subsequent heating in the neutralatmosphere of chamber 15.

It is to be understood that the curves F, G and I are for comparisonpurposes only, the exact shape and position thereof being dependent uponthe nature and composition of the metal, the heating rate, the ametemperature of the gas, the extent to which combustion is completedbefore contact with the metal, and other complex variables. The area 45representing the amount of oxidation obtained in chamber 13 may bevaried by the distribution of the air through the slots 39 and, ifdesired, the protection may be extended further towards the charging endof the furnace, thereby even further reducing oxidation in the initialheat zone. The condition illustrated by curve I, however, represents asubstantially complete freedom from oxidation and is readily obtainablewithout any material reduction in the high heating rate inherent inheating by complete combustion directly in the heating chamber. Itshould be remembered that only about 25% of the available heat in thefuel was released by the partial combustion in chamber 15 and that thegas introduced into chamber 18 will, therefore, have a B. -t. u. contentavailable for release in chamber 18 of the order of 75% of the B. t. u.Value of the fuel, the 25% released in chamber 15 'being offset in alarge degree by the fact that the gases entering chamber 18 will have atemperature of not less than 2000 F. The' temperature produced by thesecondary combustion in chamber 18 Will, therefore, be of the same orderas that which would be obtained by combustion of an equivalent amount ofcold fuel and air therein. Therefore, the

heating rate in chamber 18 will be substantially equal to 7 vents 36provided in the end wall 17. The portion of chamber 18 more remote fromthe slot 38, where the Work is at o'r above scaling temperature', isenlarged so that sufficient combustion space is provided above the'-Vents 46 are normally closed; however, and chambers 15 and 18 are ventedonly through the vents Vents 36 and 46 are each provided at their upperopen ends With hoods 47 and 48, respectively, the outlet ports of whichare normally closed by dampers 49 and 51, respectively. Air inletnozzles 52 and 53 are also provided for the hoods 47 and 48,respectively, so arrarged in relation to the hood outlet ports as toproduce a suction or venturi efiect when supplied with air through theirrespective valves 54 and 55.

The combustion products in chamber 18, in their passage in intimatecontact with the cold work, as they exit from this chamber will bereduced to a temperature which may be as low as 2000 F. In order tomaintain the proper venting of these gases from the furnace, the vents36 are provided with air injector nozzles 56 for producing a controlleddraft in the vents. This air also assists in cooling the gases to atemperature at which they may safely be passed through a metalrecuperator 57, the purpose of which is to preheat the air required forthe burners 28.

The recuperator 57 comprises a shell having therein a header 58connected to an incoming ajr Conduit 59, a header 61 connected to a pairof outlet air conduits 62 and a series of tubes 63 connecting theheaders 58 and 61. Conduits 62 extend from the recuperator to the innerwall passages 33 leading to buruers 28. The vented gases pass from theflue 36 into one end of the shell 57 and are exhausted from the oppositeend through ports 64. Hoods 65 are disposed over the ports 64 and areprovided with air injector nozzles 66 for augmenting the draft throughthe furnace. The aggregate amount of air supplied to the draft producingnozzles 56 and 66 will depend upon the draft required to maintain thechamber 15 at a slight positive pressure relative to chamber 18. Thedivision of this air between the two sets of nozzles will be determinedby the exact temperature of the gases vented from chamber 18. Ifmaterially above the safe Operating temperature of the recuperator, asuficient Volume of air will be supplied to nozzles 56 to cool the gasesto the desired temperature of about 2000 F. and only enough air will besupplied by nozzles 66 to boost the draft to the desired degree.However, if the exhaust gases are at or below the desired temperaturelittle or no air will be supplied to the nozzles 56 and the draft willbe created primarly by ncreasing the air supply 'to nozzles 66. Thus,the preheat temperature of the air supplied to the burners 28 may bemaintained at its desired level, preferably at a temperature of about1000 F.

As previously stated, the damper valves 49 at the upper end of vents 36are normally closed and all of the furnace gases are vented through theheat exchanger 57. All or a portion of these gases may be diverted fromthe heat exchanger, if desired, by opening of damper 49 and air injectorvalve 54.

Damper 51 and injector valve 55 associated with the vents 46 at thedischarge end of the furnaces are provided for the purpose of relievingthe positive pressure of the furnace gas adjacent slot 21, when the door27 is momentarily opened to discharge work from the furnace, and therebyprevent or reduce burning of the rich atmosphere about the door opening.p These vents also serveto prevent an inux of air into chamber 15 atsuch times.

The control of injector valve 55, damper 51, door 27 and work ejector 24at the discharge end of the furnace 8 and the interlock between doors 26and 27 Will now be explained with reference to Fig. 5.

The control mechanism disposed -at the charging end of the furnacecomprises an electrically operated reversing valve 67 for supplying airunder pressure to the pneumatic cylinder 68, the function of which is toopen and close the charging door 26 through the chain 69; a signal lamp71 for indicating when the discharge door 27 is open; and a controlswitch 72. At the discharge end a similar reversing valve 73simultaneously controls pneumatic cylinder 74 to operate the workejecting levers 24 and cylinder 75 to open the discharge door 27 throughlinkage 76. Damper 51 is connected to the piston of cylinder 75 throughresilient linkage 77 so as to open whenever door 27 is opened. A signallamp 78 and switch 19 are also provided at the discharge end of thefurnace. When the operator at the charging end desires to open the door26 -for insertion of -a work piece, he depresses switch 72`,interrupting the grounded circuit through the upper contacts andcompleting a battery circuit at the lower contacts. This latter circuitextends in parallel through the winding of the reversing valve 67 toopen door 26, and the signal lamp 78, to indicate the open charging doorcondition at the discharge end of the furnace, the circuit beingcompleted through the upper contacts of switch 79 to ground. Thus, thiscircuit can only be completed when switch 79 is in its normal doorclosing position. Switch 79 in its lowered position interrupts theabove-mentioned ground and completes a battery circuit (l) in parallelto the reversing valve 73 to supply `air to the cylinder 74 to operateejector 24, and to supply air to cylinder 75 to open door 27 and actuatedamper valve 51; (2) to ejector valve 55 to supply, -air to the nozzle53 whereby to produce a suction in the exhaust vents 46; and (3) tosignal lamp 71 at the charging end of the furnace. This circuit isgrounded through the upper contacts of switch 72 and hence can only becompleted when switch 72 is in its normal door closing position. Thisprovides an nterlock whereby only one door may be open at any time,thereby minimizing interference With the protective atmosphere blanketby the door opening Operations.

It is to be understood that the furn-ace may be provided with the usualtemperature controls and with appropriate zoning of the various burners.In this respect, it is contemplated that slots 39 will normally provideonly sufficient` air to produce complete combuston of the gasesentered'into chamber 18. Howeve', should the temperature of the workleaving chamber 18 tend to exceed the desired upper limit, 2000 F. inthe example given,

excess air may be added through the slots 39 to resto-e the desiredtemperature condition in this chamber. If the work fails to attain itsdesired temperature in chamber 18, this will be reflected in chamber 15,as the Work passes therein, so that extra heat Will be called for inthis chamber. The increase in gas thus supplied to chamber 15 will serveas additional fuel for chamber 18 so as to increase the temperature inchamber 18 until the proper heat balance is established. Controls bywhich these conditions may be attained are well known in the art and,therefore, have not been included herein.

Obviously, the process is subject to many variations without departingfrom the essential features of the invention and may be carried out invarious other types of furnaces. Therefore, I do not desired to belimited to the details of the specific disclosure of this applicationexcept as required by the terms of the appended claims.

What I claim is:

1. A furnace for the neutral heating of metal comprising a heatingchamber having a hearth, an inlet opening at one end of the chamberadjacent said hearth for the admission of work and an outlet opening atthe opposite end of said chamber adjacent said hearth for the removal ofwork, a pluralty of burner means extending into said chamber. atspacedpoints along a portion there.-

of adjacent the outlet opening separate air and fuel supply conduitsextending into said burner for the production and entrance of prima'ygaseous products of reaction of a hydrocarbon fuel and air in whichthere is a large deciency of air for complete combustion, venting meansfor said chamber adjacent said inlet opening for the discharge of saidproducts from said chamber, means for directing said vented productsinto contact with said air supply conduit, draft producing means for'said Venting means, means, including other draft producing means, fordirecting said vented products out of contact with said air supplyconduit, a plurality of air inlets spaced along a portion of saidchamber adjacent said inlet end and located at progressively increasingdistances from said venting means for adding supplemental air to saidchamber at spaced points along the path of travel of said productstoward said venting means, said air inlets being disposed above the pathof travel of said reaction products whereby to ntermingle with andeffect progressve burning of the reaction products as they approach saidventing means, and means for supporting work in transit through saidchamber in the path of travel of said primary reaction products.

2. A furnace for the neutral heating of metal comprisng a heatingchamber having an inlet opening and an outlet opening, a hearthextending therethrough from the inlet opening to the outlet opening forthe transit of work through the chamber, a stepped roof for said chamberhaving a low section extending from a medial point toward said outletend and a relatively higher section extend'ng from said medial pointtoward said inlet end, whereby to form a final heating zone' and aninitial heating zone, means for introducing heated combustion productsof an air deficient mixture of fuel and air into said final heatingzone, venting means for the chamber disposed adjacent the inlet opening-at a level above the hearth approximately on an extension of the roofline of said final heating zone, suction producing means for saidventing means whereby said combustion products will be drawn along saidhearth without substantal change of direction from said final heat zonethrough said initial heating zone to said venting means to thereby forma protective blanket o ver said work, means for progressively addingsupplemental air to said chamber in said initial heating zone at a levelsubstantially above said roof line to produce progressive burning ofsaid combustion products at the upper surface of said blanket as theypass to said Venting means, a door normally closing said outlet opening,a normally closed vent for said final heating zone disposed adjacent tosaid door and means for concurrently opening said door and vent.

References Cited in the file of this patent UNITED STATES PATENTS

